The present invention relates to a synthetic mixed-layer silicate or, more particularly, to a synthetic mixed-layer silicate having serpentine and smectite as the component layers as well as a method for the preparation thereof. To say further, the invention relates to a synthetic mixed-layer silicate capable of being dispersed in water to form a gel and capable of giving a porous material by dehydration of the aqueous gel as well as a method for the preparation thereof by a process including a hydrothermal treatment.
Occurrence in nature is well known of various kinds of mixed-layer minerals including those consisting of mica and smectite as an example. See, for example, Crystal Structure of Clay Minerals and their X-Ray Identification, edited by G. W. Brindley, et al., Mineralogical Society, London, pages 249-303 (1980). They are scarcely used as industrial materials because of their rare occurrence in nature so that almost no attention has been paid to the practical application of such mixed-layer minerals excepting for a small amount of raw materials of pottery. Nevertheless, most of mixed-layer minerals belong to the class of very unique materials having an interstratified structure consisting of a combination of non-swellable layers and swellable layers possibly to exhibit characteristics of both of these constituents so that it is eagerly desired to develop a novel functional material by controlling the interstratified structure thereof.
For example, so-called 1:1 clay minerals can be classified into two major groups including, one, kaolinite which is a dioctahedral clay mineral having aluminum atoms in the octahedral sheet and, the other, serpentine which is a trioctahedral clay mineral having magnesium atoms occupying the octahedral cation sites. Known mixed-layer minerals consisting of layers of a 1:1 clay mineral as the non-swellable layers and smectite layers as the swellable layers include those consisting of kaolinite and smectite. See, for example, X-Ray Diffraction and the Identification and Analysis of Clay Minerals, D. M. Moore, et al., Oxford University Press, pages 259-263 (1989). While a mixed-layer mineral consisting of serpentine and smectite is supposedly a possibility of mixed-layer minerals, no report is found of a mixed-layer silicate of this type.
Serpentine occurs in nature as a main constituent mineral of serpentinite formed by the metamorphic transformation of olivine and is classified into three types including chrysotile having a fibrous morphology, lizardite having a morphology of platelets and antigorite having a morphology of platelet accompanied by a wavy superstructure in the direction of the x-axis. Aluminum lizardite and amesite are each a mineral formed by isomorphous substitution by aluminum atoms in the tetrahedral sheet and octahedral sheet of serpentine and have a morphology of platelet.
Serpentine has a typical chemical composition expressed by the general formula EQU Si.sub.2-a Al.sub.2a Mg.sub.3-a O.sub.5 (OH).sub.4,
in which the subscript a is 0 or a positive number not exceeding 1. Serpentine minerals are classified by the value of this subscript a including lizardite having a value of a equal to or close to 0, amesite having a value of a equal to 1 and aluminum lizardite having a values of a intermediate between 0 and 1.
Aluminum lizardite can be synthesized by the hydrothermal reaction as is reported in Am. Miner., volume 44, pages 143-152 (1959). Serpentine minerals inherently do not exhibit layer charges so that they have no cation-exchangeable capacity. They are usually not dispersible in water so that sol or gel can never be formed therefrom. Among the serpentine minerals having a fibrous morphology, chrysotile is used in various applications as asbestos. Almost no industrial applications have been developed, however, for the serpentine minerals of other types.
On the other hand, smectite is a clay mineral consisting of very fine particles and smectite minerals in general have unique properties such as ion-exchangeability exhibited by the layer charges, swellability in water, dispersibility, intercalation and the like so that, different from other clay minerals such as kaolinite which is used as a raw material in pottery, they are used in various special industrial applications such as drilling mud, foundry sand, starting material in the preparation of organophillic smectites as an additive in paints, printing inks and greases, and so on.
Smectites are grossly classified into the dioctahedral smectites and trioctahedral smectites having, in many cases, trivalent aluminum atoms and divalent magnesium atoms, respectively, as the octahedral cations. Substitutions by cations of lesser charges, notably Si.sup.4+ by Al.sup.3+ in tetrahedral sheets and Al.sup.3+ by Mg.sup.2+ in octahedral sheets, produce resultant negative layer charges on the layers which are balanced by interlayer exchangeable cations. The minerals belonging to the dioctahedral smectite include montmorillonite, beidellite and the like while the minerals belonging to the trioctahedral smectite include saponite, hectorite, stevensite and the like.
The saponite has a chemical composition expressed by the general formula EQU Si.sub.4-c-d Al.sub.c+2d Mg.sub.3-d O.sub.10 (OH).sub.2.(Na).sub.c,
and the layer charges are determined by the value of the subscript c obtained by subtraction of the amount of aluminum substituting octahedral Mg from the amount of aluminum substituting tetrahedral Si. A method is disclosed in Japanese Patent Kokai 58-181718 for the synthetic preparation of a saponite by the hydrothermal method at a temperature not exceeding 350.degree. C.
On the other hand, U.S. Pat. No. 3,855,147 discloses a synthetic smectite composition having a structure in which an accessory phase selected from the group consisting of magnesium oxide, magnesium hydroxide and hydrous magnesium oxide exists in the saponite-like crystal lattice. It is, however, rather likely that an artificial mineral is close to that of a chlorite-like clay mineral if the saponite-like mineral has the structure containing magnesium hydroxide or hydrous magnesium oxide within interlayers. The above mentioned U.S. patent contains an example for a synthetic smectite composition in which a single accessory phase is formed from magnesium oxide alone. At any rate, absolutely no suggestive disclosures are found in the prior art for the mixed-layer silicate consisting of serpentine and smectite as is the subject matter of the present invention.
In view of the above described situations relative to synthetic mixed-layer minerals, the inventors have conducted extensive investigations for the development of novel mixed-layer minerals or, in particular, silicates having usefulness in an industrial application arriving at the completion of the present invention.